This invention relates to a pass-through heating system for discontinuous operation as an input to vapor coating apparatus, preferably for coating metal substrates, which includes a heating chamber having at least one heating means and a thermal barrier disposed between the heating chamber walls and the heating system, parallel to the direction in which the substrates are transported.
It is known to heat substrates to temperatures that are considerably higher than room temperature, for the purpose, for example, of bringing about intermetallic diffusion to increase the strength of adhesion between the base metal of the substrate and the vapor coating. In the application of oxidation and corrosion inhibiting surface coatings by vapor coating to gas turbine blades, temperatures between about 800.degree. C. and 1100.degree. C., preferably between 950.degree. C. and 1000.degree. C., have proven especially desirable whenever alloys of CoCrAlY and NiCoCrAlY, for example, are used in producing the surface coatings.
From German Pat. No. 19 41 254, it is known to provide for the heating of the substrates in the actual vapor coating chamber by installing heat sources by which the substrates can be heated to temperatures of about 980.degree. C. Aside from the necessity of preventing the condensation of metal vapors on the heat sources themselves, this also ties up the vapor coating chamber for the duration of the heating cycle, thereby adversely affecting the economy of operation of the entire apparatus. Acceleration of the heating process by increasing the heating power is impossible because the propagation of heat into the substrates can take place only by thermal conduction, and excessively great heating power would entail the danger of local overheating.
It is also known to heat substrates in preheating chambers, in which case, however, the substrate temperatures have not exceeded maximums of about 300.degree. to 400.degree. C. For this reason, it has not been necessary to equip the preheating chambers with heat barriers making make them into virtually fully enclosed chambers. When the temperature level is so low, local "sinks" or sources of heat losses do not play the same important part they play at a high temperature level.
Whereas in general it is not at all difficult to bring a single substrate to a precisely predetermined temperature level if the heating time is sufficiently long, the difficulties increase substantially when a plurality of substrates are involved which are spread out in a plane for vapor coating and have to be heated at a uniformly high temperature level. The simultaneous coating of a plurality of substrates, however, is essential to the large-scale application of the coating processes involved. These are vacuum depositing processes, cathode spray processes, the chemical vapor deposition process, ion implantation, etc.
In the production of the oxidation and corrosion inhibiting coatings it has been found that a substrate temperature of, for example, 1000.degree. C. must be maintained in all substrates with very great precision, i.e., within no more than .+-.10.degree. C. To satisfy such requirements, it is known to provide pass-through heating with systems an appropriate length, i.e., about twice as long as the length of the substrates. Such an increase of length, however, has the disadvantage in the case of vacuum apparatus that it has to be doubled, because the transport rack that is provided for the substrate holder has to be made correspondingly longer. Since the transport rack has to be withdrawn into a lock chamber or loading chamber, it is necessary to make this chamber correspondingly longer, and this is the reason for the doubling of the structural length.